Dissolving a volatile fraction in a liquefied gas

ABSTRACT

A volatile gas such as nitrogen is dissolved in a liquefied gas such as natural gas, by providing a relatively small interface between the vapor and the liquid at a low flow rate of the vapor toward the liquid, and a relatively large interface at a relatively high flow rate of the vapor. This is done by introducing the vapor and the liquid into an upright conduit in which is disposed a spherical container for the liquid. The spherical container has a perforated bottom. A low vapor flow rate, the liquid immerses the container and the interface is small; but at a high vapor flow rate, the liquid level in the conduit is forced down below the perforations of the container, and these exposed perforations establish jets of liquid through the vapor, thereby increasing the interface.

United States Patent [191 Cappiello [111 3,830,073 1 1 Aug. 20, 1974 1 DISSOLVING A VOLATILE FRACTION IN A LIQUEFIED GAS [75] Inventor: Pierre Cappiello, Paris, France [22] Filed: July 12, 1971 [21] Appl. No.: 161,461

[30] Foreign Application Priority Data Primary ExaminerNorman Yudkoff Assistant ExaminerF. Sever Attorney, Agent, or FirmYoung & Thompson; Irvin S. Thompson; Robert J. Patch [57] ABSTRACT A volatile gas such as nitrogen is dissolved in a liquefled gas such as natural gas, by providing a relatively small interface between the vapor and the liquid at a low flow rate of the vapor toward the liquid, and a relatively large interface at a relatively high flow rate of the vapor. This is done by introducing the vapor and the liquid into an upright conduit in which is disposed a spherical container for the liquid. The spherical container has a perforated bottom. A low vapor flow rate, the liquid immerses the container and the interface is small; but at a high vapor flow rate, the liquid level in the conduit is forced down below the perforations of the container, and these exposed perforations establish jets of liquid through the vapor, thereby increasing the interface.

1 Claim, 2 Drawing Figures gas. It concerns more particularly the dissolution of gaseous nitrogen in liquefied natural gas.

There has been described in US. Pat. application No. 837.5 18, filed June 30, 1969, refiled as Ser. No. 239,l36, Mar. 29, 1972 a method of producing from liquefied natural gas, a gas rich in methane to which is added a volatile gaseous fraction. One stage of this method consists in condensing the volatile fraction by mixing it with liquefied natural gas at a given pressure. The volatile fraction may also be mixed in two-phase or liquid form.

During this addition in gaseous from, it is necessary intimately to admix the gas and liquid so as to increase the exchange surface and favor the thermal and mass transfers.

The present invention proposes to provide a satisfactory solution for these thermal and mass transfer problems in the case of dissolving a volatile fraction in a liquefied gas.

The invention consists in a method comprising contacting a said liquefied gas with a said volatile fraction at least partly in a vapor phase over a first vapor liquid interface having a first relatively small area at a first relatively small flow rate of said vapor phase toward said interface, and increasing both said flow rate and said area.

According to one embodiment of the invention, the larger interface is established by establishing a plurality ofjets of said liquefied gas passing through said vapor phase.

The invention also relates to apparatus for dissolving a volatile fraction in a liquefied gas, comprising an upright conduit, means for introducing the volatile fraction at least partly in vapor phase into said conduit, means for introducing liquefied gas into said conduit and for establishing a first relatively small vapor-liquid interface between said vapor phase and said liquefied gas in said conduit at a first relatively low flow rate of said vapor phase into said conduit, and means for establishing a second relatively large vapor-liquid interface between said vapor phase and said liquefied gas at a second relatively high flow rate of said vapor phase into said conduit.

The apparatus according to the invention enables extremely flexible operation. On the one hand, it is possible to mix the volatile fraction with the liquefied gas whatever its physical state, i.e. gaseous or two phase. On the other hand, when the volatile fraction is mixed in gaseous form, the contact surface between the gas and the liquid necessary for thermal and mass transfer itself adapts to the ratio of the gaseous flow to the liquid flow passing into the apparatus. Thus, for a given liquid flow, if the gaseous flow increases, the contact surface increases.

In order that the invention may be more clearly understood reference will now be made to the accompanying drawings which illustrate one embodiment thereof by way of example, and in which:

FIG. 1 shows a view in section along the line B-B of FIG. 2 of apparatus in accordance with the invention, the ducts and valves for effecting the supply and evacuation of the apparatus being shown diagrammatically, and

FIG. 2 shows a view in section along the line A-A of FIG. 1 of the same apparatus: the fluid inlets and outlets are also shown diagrammatically on this Figure.

Referring now to the drawings, the apparatus according to FlGS. l and 2 comprises primarily two parts, viz: a vertical conduit or column I and another conduit or vessel 2. The column 1 is substantially cylindrical; the vessel 2 is substantially cylindrical and slightly inclined with respect to the horizontal. The column 1 and the vessel 2 intercommunicate at their intersection and form a sealed unit. The upper part of the column 1 is closed by a spherical dome 3. A spherical chamber 4 coaxial with the dome 3 is arranged within this upper part within and at a distance from the wall of the column 1. The lower part of the chamber 4 is perforated at 5 or is provided with other means connecting its interior and the column 1. These perforations 5 are distributed in the wall of the sphere 4 over part of the spherical surface. Two cylindrical coaxial tubes 6 and 7 are arranged along the axis of symmetry and in the upper part of the column 1. The conduit 6 enabling the chamber to be supplied with liquefied gas, communicates with the interior thereof; the conduit 7 enabling the volatile fraction in gaseous form to be introduced into the column 1, communicates with the interior thereof. A horizontal annular plate 8 shuts off the conduit 7 while a radial pipe 9 communicates with the interior of the conduit 7. A radial pipe 10 open at its end 11 passes into the column 1 below the chamber 4. A tube 12 provided with a valve 13 communicates with the upper part of the interior of the column 1 and enables periodic evacuation of the insoluble and noncondensable gases. The vessel 2 is provided internally at its median part with means for injecting the volatile fraction in gaseous form, and comprising two transverse partitions 14 and 15 adjacent to one another and attached to the wall of the vessel 2. These partitions 14 and 15 carry a rectangular perforated plate 16 and form a box 17 communicating with the interior of the vessel 2 through said perforations. The perforated plate 16 is substantially horizontal and is close to the axis of symmetry of the vessel 2.

Two exterior cylindrical conduits l8 and 19 arranged on both sides of the vessel 2 communicate with the interior of the box 17 below the perforated plate 16. The conduits 18 and 19 have the same axis of symmetry; the latter is perpendicular to the axis of symmetry of the vessel 2 and is contained by the perforated plate 16. The conduits l8 and 19 are closed at their ends by cylindrical plates 20 and 21 respectively. Two radial and vertical pipes 22 and 23 communicate respectively with the conduits l8 and 19. A conduit 51 communicates with the interior of the vessel 2 at its base.

The spherical chamber 4 is supplied with liquefied gas through a connection 24 linked to the tube 6 and controlled by a valve 25. The column is supplied with volatile fraction in gaseous form by a pipe 26 connected to the tube 9 and controlled by a valve '27. The vessel 2 is supplied with volatile fraction in gaseous form through pipes 28 and 29 connected to tubes 22 and 23 respectively. The column is supplied with volatile fraction in liquid form through a pipe 30 connected to the tube 10 and controlled by a valve 50. The liquefied gas of modified composition is removed from vessel 2 by a pipe 52 connected to the tube 51 and controlled by a valve 53. The pipes 28 and 29 communicate with a pipe 31 controlled by a valve 32. The pipes 31 and 26 communicate with a pipe 33. The pipes 30 and 33 communicate with an input pipe 34 for the volatile fraction in gaseous or two-phase liquid-vapor form.

Means 35 for detecting the physical state of the volatile fraction flowing through the pipe 34 are arranged thereon upstream of the intersection of the pipe 30 and 33. The information supplied by the means 35 is transmitted to a control box 36 which may actuate the valve 50 and the valves 27 and 32.

In order to describe the operation of the device according to FIGS. 1 and 2, reference is made to the addition of nitrogen to liquefied natural gas. In order to illustrate the flexibility of operation of the apparatus, for a constant mass flow of liquefied natural gas, the case of a dissolution of an increasing mass flow of gaseous nitrogen and the case of a dissolution of a constant mass flow of nitrogen the temperature of which is decreasing, are considered in succession.

In the first case, a constant flow of liquefied natural gas passes into the spherical chamber 4 via the pipe 24 and the open valve 25. The valves 50 and 32 being closed, and the valve 27 being open, gaseous nitrogen is injected into the column 3 through the conduit 9. If very little nitrogen is injected, the level of liquid establishes itself, for example, at the level P between the wall of the column 1 and the chamber 4 at the height of the upper hemisphere of the chamber 4, and the small contact surface obtained corresponds to the small mass and thermal transfers required for the dissolution of the gaseous nitrogen. If the gaseous flow is increased, the contact surface is lowered and increases between the horizontal plane P and P thus adapting the increasing flow of gaseous nitrogen. Then from the level P the perforations 5 of the sphere 4 are uncovered and the liquefied natural gas is thus distributed in a spray or in a plurality of liquid jets. From P as the nitrogen flow increases, the level of liquid in the column 1 falls, for example to P Between P and P, the contact surface corresponding to the horizontal gasliquid interface is increased by the surface of the liquid jets leaving the chamber 4. The latter surface also increases when the flow of nitrogen increases. From the gaseous flow corresponding to the establishment of the level of liquid at P, the valve 32 is open and the supplementary flow of gaseous nitrogen is injected into the box 17 and distributed across the perforated plates 16 in the liquid current contained by the vessel 2. For small additional flows, injected by the box 17, nitrogen is dissolved completely in the liquid current of the vessel 2. When the supplementary flow is increased, the liquid current from the vessel 2 is saturated with nitrogen and the non-dissolved gaseous part moves up in the vessel 2 and the column 1. The level of the liquid thus establishing itself between P, and P and the residual nitrogen issuing from the vessel 2 is dissolved on meeting the liquid jets issued from the sphere 4.

According to the foregoing, it is thus possible to adapt the contact surface required for the dissolution of nitrogen in the liquefied natural gas to the actual flow of gaseous nitrogen.

In the second case considered, a constant mass flow of gaseous nitrogen, the temperature of which is decreasing, is injected through the conduit 9, the valves 32 and 50 being closed, and the valve 27 being open. The level of liquid establishes itself, for example at P and the liquefied natural gas is introduced into the column l in a plurality of liquid jets. When the nitrogen becomes two-phase, the mass flow to be condensed is smaller and the level of liquid in the column 1 rises. Then, when the nitrogen is liquid, the column 1 is entirely full. If the introduction of liquid nitrogen into the liquefied gas through the tube 9 is prolonged, there is a risk of forming solid deposits in the chamber 4 which may block the perforations 5. Thus, as soon as the nitrogen becomes liquid, the detector 35 closes the valve 27 and opens the valve 50 via the control box 36. The nitrogen is thus injected through the tube 10 under the chamber 4 into the liquefied natural gas still in the column 1.

The two cases of operation which have just been considered only from examples of possible operation of the apparatus according to the invention. Numerous other methods of operation can be envisaged. The nitrogen may, for example, be available in two-phase form, in which case, it is separated in a separator upstream of the column 1 and the gaseous phase is sent by the tube 9 and/or the box 17, the liquid phase by the tube 10.

By way of example, in order to form a natural gas mixed with 9.5 percent by volume of nitrogen and 2 percent by volume of natural gas formed by evaporation from a reservoir of liquefied natural gas, apparatus according to the invention is used to add these volatile fractions in the gaseous state to the natural gas in the liquid state.

In this case, the apparatus operates at R.R. ATM- .ABS, and the liquefied natural gas enters the tube 5 at 158C. The gaseous nitrogen and the evaporation of natural gas are united and introduced through the tube 9 and the pipes 22 and 23 at l42C. Of the 9.5 percent of nitrogen, 7.5 percent pass through the conduit 9 and 2 percent through the box 17. The liquefied natural gas having dissolved and condensed the volatile fraction is evacuated by the pipe 51 at -l49C.

The present invention may be used with advantage when it is desired more particularly to dissolve a gaseous volatile fraction in a liquefied gas. Naturally, the present invention is not limited to the embodiment described and shown, since it is capable of various modifications, which will be apparent to those skilled in the art, according to the applications envisaged but without, however, departing from the scope of the invention.

What we claim is:

1. A method for dissolving continuously a volatile fraction in a liquefied gas unsaturated with said volatile fraction, comprising:

a. distributing a flow of said liquefied gas through a nozzle having a spherical bottom of downwardly decreasing cross-sectional area and perforations opening through said bottom, at a mean level into a vertically-extending upper portion of a sealed unit, injecting a flow of said gaseous volatile fraction into said upper portion at a level higher than said mean level in surrounding relationship to said spherical bottom of said nozzle, and removing a flow of liquefied gas of modified composition from a lower portion of said unit, whereby there is established inside said unit a contact surface for heat and mass transfer between said gaseous volatile fraction and said liquefied gas, comprising a horirounding region at a plurality of different levels so that progressively more perforations are exposed as said contact surface at the gas-liquid interface falls along said bottom of said chamber, in order to form a liquefied gas of determined composition in said volatile fraction. 

